JP3679508B2 - Sintered parts manufacturing method and apparatus - Google Patents

Abstract

A process for producing sintered parts with high wear resistance and good dynamic strength properties from formed bodies, which have been pressed as green parts from a completely-alloyed air-hardened heat-treatment steel powder with a carbon content of at least 0.3% added in the form of graphite. The process includes sintering the parts under protective gas at a sintering temperature of at least 1000 DEG C. and subsequent cooling. The sintered parts are cooled immediately after sintering from the sintering temperature to a first holding temperature in the range of Ar3 to a maximum of 150 DEG C. above Ar3 and are held for a first holding period of 5 to 25 minutes at this temperature (austenitizing phase). Immediately after this, the sintered parts are cooled in accelerated fashion to a second holding temperature by convective gas cooling and are held at this temperature for a second holding period. The second holding temperature lies in a temperature range in which a bainitic structure forms and is of such a length that a bainitic structure portion of at least 50% is established. The sintered parts are then cooled to room temperature.

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a sintered part having a high dynamic resistance and at the same time a good dynamic strength from a pressed shaped body according to the preceding paragraph of claim 1 and an apparatus for carrying out this method.
[0002]
[Prior art]
For example, steel mechanically loaded structural parts such as gears for transmission gears must not only have high dimensional accuracy, but also have both very good dynamic strength and high wear resistance. Must have. The process of producing such parts by cutting finish and subsequent calcination has long been considered as the only possible method. However, it is also possible to use powder metallurgical methods to reduce the molding costs. In this connection, graphite powder and conventional lubricant are added to a diffusion alloyed and oil-quenched steel powder, and then a green compact is formed from this steel powder into a powder preform. It is known that the green compact is sintered in a furnace in a continuous manner and then cooled to room temperature. In order to increase the dimensional accuracy, the pressing process is then performed again with a calibration press. Next, quenching accelerating in an oil bath is performed, followed by tempering. The structural part produced in this way has a typical tempered structure.
[0003]
Such manufacturing methods provide structural components that have good static strength (tensile strength, hardness, wear resistance) and good dynamic strength. Despite the increased cost due to the second pressing step (calibration), dimensional accuracy and uniformity are often not satisfactory. The reachable tolerance class is about IT10.
[0004]
Furthermore, it is known to produce sintered parts from green compacts pressed from steel powder that has been alloyed and air hardened. In this case, the martensite structure is formed by cooling the air to a temperature lower than the martensite start temperature. Such sintered parts certainly have high wear resistance due to their high hardness, but are suitable for dynamic loads that occur regularly, for example in gears, due to their low elongation at break. Absent. Also in terms of reachable dimensional accuracy (tolerance class IT9), the sintered parts produced in this way are not satisfactory.
[0005]
Finally, from German Patent Application Publication No. DE 40018999C1, a high strength sintered part powder consisting of steel powder which has been subjected to alloy processing and which has a carbon content ratio of 0.3-0.7% is added. In order to produce preforms, it is known to press, sinter at temperatures in the region of 1120-1280 ° C., quench by cooling and then temper.
[0006]
[Problems to be solved by the invention]
The object of the present invention is to maintain the good dynamic strength and high wear resistance, greatly increase the dimensional accuracy (narrow finish tolerance), and keep the processing and equipment costs as small as possible. The method is to improve. Furthermore, it is providing the apparatus which implements this method.
[0007]
[Means for Solving the Problems]
This object is achieved according to the invention by the features described in the characterizing part of claim 1. Advantageous embodiments are described in claims 2 to 8. An apparatus for carrying out the method of the present invention has the features described in the characterizing part of claim 9 and can be developed by the features described in the characterizing part of claims 10-12.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail based on the embodiments with reference to the drawings.
The basis of the present invention is to use a known tempered steel powder to produce sintered parts. Tempered steel is steel whose quality is adjusted by tempering, and is manufactured from the finished alloy steel. The distribution of alloy components in the tempered steel powder is uniform except for the C content. This uniform distribution is therefore not obtained for the first time by achieving diffusion during sintering for a long time.
[0009]
In the past, sintered parts were heat treated separately after the sintering process, which provided the sintered parts with good dynamic strength properties and wear resistance. Such adjustment of characteristics is performed directly in the course of the sintering process. An important point for realizing this is that the steel powder used is made of an air-quenched material. This eliminates the need for quenching using an oil bath that is originally undesirable for environmental protection reasons.
[0010]
The carbon content of the sintered part is added separately in the usual manner in the form of graphite, so that the steel powder remains sufficiently soft to ensure sufficient compression moldability. During the sintering process, graphite diffuses into the connected powder particles.
[0011]
In the present invention, as shown in FIG. 1, cooling is performed from the sintering temperature to the first holding temperature directly following the sintering (section a). First holding temperature is located within a temperature range of from Ar ₃ to a temperature of Ar ₃ above the maximum 0.99 ° C.. The cooling (b section) from the sintering temperature to the first holding temperature is preferably performed at a cooling rate of 0.5 to 1.5 ° C./s. The sintered part is held at the first holding temperature for about 5-25 min (first holding time, section c). This achieves a smaller austenite grain size.
[0012]
As is well known to those skilled in the art, Ar ₃ is a critical temperature or transition temperature when cooling steel, and is a temperature that varies depending on the steel components.
[0013]
In the austenitization phase (section c), the following is recommended: to further enhance the C-Potential in a protective gas atmosphere that had to be maintained throughout the sintering process Is recommended. As a result, the outer surface layer of the sintered part is carburized, thereby making it possible to significantly increase the hardness in the surface region. This is important for good wear resistance. On the other hand, the carbon content is kept low inside the sintered part, which results in very good dynamic strength properties (cross section hardness distribution).
[0014]
It is particularly advantageous if the first holding temperature is selected in the region of maximum 50-100 ° C. above Ar ₃ . A suitable duration of the first holding time is 10-20 min.
[0015]
Immediately following the first holding time, cooling is accelerated by gas cooling by convection to reach the second holding temperature (d section). In order to achieve this, a cooling rate in the region of 3-6 ° C./s is recommended.
[0016]
This second holding temperature is selected based on the ZTU chart of each material so that the region of ferrite formation is avoided and the bainite structure begins to form.
[0017]
This second holding temperature holding time (e interval) continues until at least 50% of the bainite texture component is formed. However, it is usually undesirable for the structure to completely convert to bainite. It is preferable to end the holding of the second holding temperature when the bainite reaches a maximum of 95% as the latest time. Particularly preferably, the proportion of the bainite component is on the order of 60 to 80%. The sintered part is then cooled to room temperature as usual (normal cooling, section f).
[0018]
One surprising point that has been found is that a particularly good part quality is guaranteed by the method of the invention. That is, not only is the dimensional accuracy relatively high, but the tolerance is much smaller than in the case of the conventional manufacturing method. Instead of a quality grade IT10 that can be reached using conventional oil quenching and tempering, IT8 can be reached in the present invention. This is even more surprising since it is not even necessary to perform a separate calibration operation. This eliminates the need for one complex and large-scale work process. Furthermore, energy costs and handling labor for a separate heat treatment process are not required.
[0019]
FIG. 2 shows the simplest arrangement of the apparatus according to the invention which is formed as an electronically controlled continuous sintering furnace. The arrow on the left indicates that the sintered part is introduced into the first zone. The first zone functions as a heating zone in which the lubricant (e.g. wax) contained in the powder preform evaporates. This first zone is therefore also called dewaxing zone 1.
[0020]
The original sintering zone 2 is arranged directly following the dewaxing zone 1 in the conveying direction. In the sintering zone 2, the sintered part is held at the sintering temperature (at least 1000 ° C.) for a sufficiently long time.
[0021]
Since the sintered part passes through the entire device at a constant speed, the sintering zone 3 has a corresponding length.
[0022]
In order to prevent the sintered parts from being oxidized, an atmosphere containing no oxygen (protective atmosphere) is maintained in the entire apparatus.
[0023]
Following the second sintering zone 2, an austenitizing zone 3 is arranged. In the austenitizing zone 3, the sintered part is first cooled and kept at the austenitizing temperature.
[0024]
Next, a rapid cooling zone 4 is arranged. The rapid cooling zone 4 is provided with a gas shower (not shown) that realizes sufficiently strong convective gas cooling.
[0025]
When the sintered part reaches the second holding temperature, the sintered part enters the bainite zone 7 and is held at this temperature long enough for the second holding time, so that at least 50% west of bainite is Formed in the organization. In order to achieve this, the bainite zone 7 has a corresponding length.
[0026]
After sufficient bainite time has elapsed, but before reaching a component ratio of 95% as much as possible, the sintered part enters the finished normal cooling zone 5 where it is from room temperature to room temperature. It is cooled to a temperature close to.
[0027]
FIG. 3 shows a device which is modified with respect to FIG. The difference of this device is that the powder preform loaded in this device can selectively travel in two different passages.
[0028]
The configuration from the dewaxing zone 1 to the rapid cooling zone 4 is completely consistent with the configuration shown in FIG. The direction of material flow can be selectively set behind the rapid cooling zone 4.
[0029]
The formed sintered part directly enters a separate normal cooling zone 5a and exits from this device as a "normally" sintered part, i.e. formed as a part not according to the invention, or according to the invention. In order to apply the method, parallel to the first part of the apparatus of the present invention, as indicated by the arrows shown, via a lateral transfer device 6 which can be selectively switched after leaving the rapid cooling zone 4. Are introduced into the bainite zone 7, which is arranged in FIG.
[0030]
Preferably this transport direction is opposite to the first part of the overall apparatus of the present invention.
[0031]
Then the normal cooling zone 5b is again arranged, in which the parts treated according to the invention are cooled and reach room temperature. Thus, such a device provides a high degree of flexibility in terms of the variety of products to be processed.
[0032]
Of course, it is also possible to dispose the bainite zone 7 and the second normal cooling zone 5b so as to be shifted from each other by 180 °, that is, to maintain the original material flow direction.
[0033]
Similarly, the normal cooling zone 5a and the apparatus row formed from the bainite zone 7 and the normal cooling zone 5b can be easily replaced with each other. However, the illustrated example has the advantage of having a relatively short structural length.
[0034]
【Example】
The effects of the present invention will be described in detail based on the following two examples.
Comparative example:
A steel powder having a composition of Fe-4Ni-0.5Mo, which has been subjected to an alloy treatment, and has a density of 6.80 from the addition of elemental elements 1% Cu, 0.6% graphite and a normal lubricant. A green compact of ˜6.90 g / cm ³ was produced. These parts are sintered at a temperature of 1150 ° C. for 30 minutes. At that time, a protective gas atmosphere consisting of an endothermic atmosphere and having a controlled carburizing ability was maintained. When the part is gas cooled by convection at the martensite start temperature (cooling rate of 3-6 ° C./s) and then cooled by normal cooling to room temperature, the part has the following characteristics:
[0035]
Tensile strength 650 N / mm ² ,
Hardness level 550-700HV1,
Elongation at break 0.3-0.6%
The dimensional accuracy corresponded to tolerance class IT9.
[0036]
Examples of the present invention:
In the case of the above-mentioned example, the steel powder having the composition of Fe-4Ni-0.5Mo and the alloy processing is completed, and 1% Cu, 0.6% graphite and a normal lubricant are added. A green compact similar to was produced. Sintering was performed at a temperature of 1120 ° C. for a duration of 30 min in an endothermic gas atmosphere in which the carburizing ability was controlled. After austenitization, rapid cooling was performed at a cooling rate of 3 ° C./s, and further bainite was performed according to the present invention, followed by normal cooling to room temperature. At that time, a bainite structure having the following characteristics was formed in the structural part.
[0037]
Tensile strength of 750~800N / mm ^2,
Hardness level 350-450HV1,
Breaking elongation A3 up to 6%,
The dimensional accuracy of the parts produced according to the present invention was significantly better. This dimensional accuracy corresponded to IT8 class.
[0038]
The method of the present invention makes it possible to combine high toughness and high strength in a sintered structural part, and such a combination cannot be achieved with a separate heat treatment, and the present invention This method guarantees good dimensional tolerances.
[Brief description of the drawings]
FIG. 1 is a diagram showing the progress of the method according to the invention on the basis of a ZTU diagram.
FIG. 2 is a conceptual diagram of a sintering furnace for performing the method of the present invention.
FIG. 3 is a conceptual diagram of a sintering furnace for performing the method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Dewaxing zone 2 Sintering zone 3 Austenitizing zone 4 Rapid cooling zone 5 Normal cooling zone 6 Lateral conveyance apparatus 7 Bainite forming zone

Claims (12)

Pressed as a powder preform of a tempered steel powder which has been alloyed and air hardened and added in the form of graphite and having a carbon content of at least 1000 ° C. at a sintering temperature of at least 1000 ° C. In a sintered part manufacturing method of forming a sintered part comprising a molded body having high wear resistance and good dynamic strength by sintering in a protective gas and then cooling,
The sintered part is cooled in the region up to 150 ° C. above Ar ₃ to Ar ₃ following the sintering and held at this temperature for a first holding time of 5-25 min (austenite phase) ),
Directly following the aforementioned treatment, the sintered part is accelerated by cooling by convection gas to reach a second holding temperature and is held at this temperature for a second holding time, the second holding temperature. Is within a temperature region where a bainite structure is formed, and the second holding time is determined so that the proportion of the bainite structure component is at least 50%,
Subsequently, the sintered part is cooled and reaches room temperature. The sintered part manufacturing method according to claim 1, wherein the first holding temperature is 50 to 100 ° C. at a maximum above Ar ₃ . The sintered part manufacturing method according to claim 1 or 2, wherein the first holding time is 10 to 20 minutes. The method for manufacturing a sintered part according to any one of claims 1 to 3, wherein gas cooling by convection is performed at 3 to 6 ° C / s. 5. The cooling according to claim 1, wherein the cooling to the first holding temperature is performed at a cooling rate of 0.5 to 1.5 ° C./s. A method for manufacturing sintered parts. The upper limit of the first holding time is determined such that the ratio of the bainite structure component is 95% at the maximum, according to any one of claims 1 to 5, Sintered part manufacturing method. The method for producing a sintered part according to claim 6, wherein the second holding time is determined so that the bainite structure component is 60 to 80%. The calcination according to any one of claims 1 to 7, wherein the protective gas atmosphere of the austenitizing phase is set to a carburizing capacity capable of acting on the carburized portion of the sintered part. Bonded part manufacturing method. Sintering zone (2), rapid cooling zone (4) disposed behind the sintering zone (2) for gas cooling, and normal cooling zone (5) disposed behind the rapid cooling zone (4) An apparatus for carrying out the method of claim 1 comprising a sintering furnace formed as a continuous device and electronically controlled.
An austenitization zone (3) is disposed between the sintering zone (2) and the rapid cooling zone (4), and bainite is disposed between the rapid cooling zone (4) and the normal cooling zone (5, 5b). A device, characterized in that an activation zone (7) is arranged. Two normal cooling zones (5a, 5b) arranged parallel to each other are provided so as to be selectable, and one of the normal cooling zones (5b) is provided with a material via a lateral conveying device (6). And the other cooling zone (5a) is connected directly to the rapid cooling zone (4) to avoid the bainite zone (7). 11. A device according to claim 10, characterized in that the transverse conveying device (6) is arranged between the rapid cooling zone (4) and the bainite zone (7). The second normal cooling zone (5b) and the bainite zone (7) are parallel in the opposite direction to the conveying direction of the sintering zone (2), the austenitization zone (3) and the rapid cooling zone (4). 12. The apparatus according to claim 11, wherein the apparatus has a different transport direction.

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